Bus bars are used for very high currents in electrical apparatuses, and for high currents distributed throughout a building. A bus bar is a live conductor comprised of a rigid piece of copper or aluminum, usually in flat bars. For industrial applications involving electrical chases, several bus bars are pre-assembled, with or without insulators, in grounded enclosures called busways. The set of bus bars in a busway represent the phases of the chosen electrical system, much the same as insulated electrical cabling.
A particular busway, known as a “plug-in bus”, is used to distribute power down the length of a building. It is constructed to allow tap-out switches to be installed at designed places along the bus. The advantage with this scheme is the ability to remove or add a branch circuit without removing voltage from the whole circuit. An additional advantage is to protect any tapped-in electrical equipment from faults in the circuit.
Bus plugs connect to the busway to provide localized distribution to electrical appliances or devices. Circuit protection for bus plugs may be in the form of a circuit breaker or a fuse. Bus plugs often include a disconnect switch to rapidly interrupt and disconnect current flowing through an electrical device in the event of an emergency. So-called “tap boxes” are the enclosures that connect power cables feeds to a busway. A “plug-in tap box” connects to a busway with a bus plug. Bus plug sizes are graded by voltage and ampere ratings. Voltage ratings commonly range from 120/40, 208-120, 240, 277/480, 480, and 600 in the U.S. The most common bus plug ampere ratings in the U.S. are 30 amps and 60 amps, although these ratings can go as high as 600 amps.
Bus plugs are required to run under high current load for long periods of time and are often cycled on and off. Stress-of-use, under such circumstances, require the internal components and design to be robust. Component failure is limiting in the present state of art, requiring costly refurbishment or replacement. One of the components that has shown excessive wear is the electrical contacts which engage and disengage power to the equipment.
Contact design involves several elements. The material composition is critical for conductivity, as well as heating, properties. Regarding the latter, dissipating heat is one aspect while avoiding spot-welding at touch-points is another. The contact size is critical to the amount of power to be transferred through the contact surfaces, the larger surface areas-in-contact affording greater current flow. The mating force of, or, otherwise, pressure on, the contact surfaces is also critical. From a microscopic perspective, the surfaces are not flat but peaks and valleys in an undulating terrain. Pressure, often in the form of springs, forces the peaks into the valleys to increase the contact surface area.
The last design element is parallelism of the contact surfaces. Serving this element of design is what is missing in the prior art. Parallelism affects both the amount of surface area in contact, as well as a phenomenon known as “pitting”. Pitting is corrosion caused by an arcing discharge between electrodes. When two surfaces are brought together by pivoting one of the surfaces onto the other, such as is typical in the art, it is inevitable that the mating will occur through progressively narrowing angles where one portion of the surfaces will touch before another and where some angular mal-adjustment of the planes of the surfaces will remain. The gaps caused by the angular disparity of fixed contact “heads”, reduces the effective surface area while setting up, by graduated inclination, a critical distance for arcing discharge. It has been shown that up to 75% of physical contact area can lost in non-parallel contacts, notwithstanding additional loss due to corrosive effects.
The instant invention addresses this unfulfilled need in the art field of bus plugs.